NUCLEAR POWER

As reactor fleet ages, engineers ask, 'Is 80 the new 40?'

Paul Voosen, E&E reporter

Greenwire: Friday, November 20, 2009

Could nuclear power plants last as long as the Hoover Dam?

Increasingly dependable and emitting few greenhouse gases, the U.S. fleet of nuclear power plants will likely run for another 50 or even 70 years before it is retired -- long past the 40-year life span planned decades ago -- according to industry executives, regulators and scientists.

With nuclear providing always-on electricity that will become more cost-effective if a price is placed on heat-trapping carbon dioxide emissions, utilities have found it is now viable to replace turbines or lids that have been worn down by radiation exposure or wear. Many engineers are convinced that nearly any plant parts, most of which were not designed to be replaced, can be swapped out.

"We think we can replace almost every component in a nuclear power plant," said Jan van der Lee, director of the Materials Ageing Institute (MAI), a nuclear research facility inaugurated this week in France and run by the state-owned nuclear giant EDF.

"We don't want to wait until something breaks," he said. By identifying components that are wearing down and replacing them, he said, suddenly nuclear plants will find that "technically, there is no age limit."

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Indeed, as U.S. regulators begin considering the extended operations of nuclear plants -- the Nuclear Regulatory Commission (NRC) expects the first application for an 80-year license could come within five years or less -- perhaps the largest lingering question is one of basic science: How do heavy doses of radiation, over generations, fundamentally alter materials like steel and concrete?

"It's taken many years for us to understand the problem," said Gary Was, the director of the University of Michigan's Phoenix Energy Institute and an expert in aging materials. "Thirty years ago, we didn't have techniques to see these changes."

Until recently, such research has not been a priority. But within the past few years, the Department of Energy began a program looking at "long-term operations," as it is known in the industry. And provisions in the Senate's climate bill call for DOE to increase these investigations in the hope of extending plant lives "substantially beyond the first license extension period."

DOE collaborates in this research with France's MAI and the U.S.-based Electric Power Research Institute (EPRI), a nonprofit funded by many nuclear utilities. U.S. leadership in the field is natural, given the sheer age of America's reactors, many of which are already coming close to exceeding their intended operating lives.

The oldest commercial plants in the United States reached their 40th anniversary this year, and the average plant has operated for 30 years. Already, more than half of the nation's more than 100 reactors have seen their initial licenses extended for an additional two decades. Nearly all the country's plants are expected to eventually win such extensions.

As companies have encountered few hurdles toward ensuring 60 years of operation, according to one 2007 survey, a majority of executives say that it is very likely their plants will operate for 80 years or longer. It is a fairly natural progression, according to Was.

"If they last till 60, maybe they can last to 80," Was said. "Heck, maybe 100?"

Many believe that these extended life spans -- longer than almost any coal-fired plant and rivaling hydroelectric projects like the Hoover Dam -- are a necessity at a time when the vast majority of the United States' energy is generated from carbon-intensive fossil fuels. Nuclear power supplies some 20 percent of the nation's electricity.

Without the recent extensions, the electricity market would have faced a severe shortage of supply that would have been "nothing short of catastrophic" and resulted in more coal- or gas-fired power plants being built and increased greenhouse gas emissions, Was said.

"Without relicensing, we go off a cliff five years from now," Was said.

While much debate in Washington focuses on the construction of new nuclear power plants, these plants will only replace the electricity produced by existing facilities unless further extensions are sought, according to Ronald Szilard, the technical director of DOE's Light-Water Reactor Sustainability Program at Idaho National Laboratory.

"The focus right now is very intense on building new nuclear power plants, because we have come to realization that [reducing] greenhouse gas emissions in the future cannot be achieved without pushing nuclear further," he said. "Both existing and new plants will have to contribute."

The original 40-year licensing period for power plants in the United States was never a question of the technical life of the plant, added John Gaertner, the lead technical manager on EPRI's long-term operations project.

"The engineers didn't throw up their hands and say, 'That's the lifetime I can get out of these things,'" he said. "It wasn't a technical reason."

Engineers have learned much about how to operate plants, reducing the time reactors remain offline during routine operations. And, while it varies on a case-by-case basis, the "assumption is generally correct that it is getting more cost-effective" to replace plant parts, said Scott Brooks, a spokesman for the Tennessee Valley Authority, which operates several nuclear sites.

"With 30 years of operating experience, we feel we can reassess a lot of the original assumptions" involved in running plants, Gaertner said.

Cracks kill

It has been nearly 70 years since the world's first artificial fission reaction, created by Enrico Fermi and collaborators beneath abandoned athletic-field stands at the University of Chicago. In that time, scientists have come to understand much about how neutrons -- the uncharged particles that provide ballast to the atom's nucleus -- alter the composition of materials.

Neutrons are effectively the trigger for nuclear power. Each time uranium splits in a nuclear reactor, neutrons are shot out at high energies. These neutrons in turn cause more uranium splits, resulting in a self-sustaining reaction. But while causing these divides, the neutrons also relentlessly pummel the steel and other metals that enfold the nuclear reactor, known as the pressure vessel.

"There are millions of millions of millions of impacts per year. At some point, it begins to impact the reactor vessel," he added.

After some time, decades or more, the radiation causes changes to the microstructure of metals, Was said. The relentless bombardment creates minute flaws, such as dislocation loops or precipitates, that "tend to harden the material," Was said. "When it gets hard, the trade-off is ductability -- the ability to transform."

As metals lose this plasticity, they lose the ability to give way, turning brittle and becoming a breeding ground for cracks and fissures. And when it comes to nuclear power plants, cracks kill.

"Cracking, the failure of the pressure vessel, has always been one of the major issues that can limit the life of the plant," Was said. "If you can't demonstrate its integrity, you're dead in the water in terms of life extensions."

In ways not yet understood, the corrosion that eventually accompanies the pristine water used in nuclear reactors interacts with and exacerbates incipient breaks in metal alloys. Such cracks are among the primary concerns of NRC when considering the future of U.S. reactors, according to Scott Burnell, an agency spokesman.

This cracking, which can affect core components down to baffle bolts, has been studied for 30 years and has still defied explanation, scientists say. In the meantime, it has cost the U.S. nuclear industry some $10 billion due to forced outages, increased inspection requirements, component replacements and increased regulatory scrutiny, according to EPRI.

Getting a handle on age-induced cracking will be one of the principal missions of nuclear scientists and engineers over the next decade, as U.S. regulators are expected to become much more stringent for extensions to 80 years, requiring models that will predict when cracks will occur, Gaertner said.

Since it is hardly time-effective to reproduce 60 years of natural neutron exposure, scientists instead use test reactors to expose steel and other alloys -- up to 25 varieties of metals can be found in reactor systems -- to higher energy radiation that simulates the plant's conditions.

Once the simulations are complete, high-powered imaging is applied to the metals. For this reason, France's MAI has one of the world's most powerful electron microscopes. Such imaging and vast increases in computing power now allow scientists to reconstruct the millions of individual atoms boiled off the surface of a metal into something similar to a photograph. The resolutions achieved allow unprecedented insights into cracking metals, Was said.

"We have a better understanding of how they form and why they form," Was said. "Once we know that, we can use that knowledge to predict into future how hardening will occur and what potential risks are."

Unanswered questions

There are also risks that could be wholly unanticipated, or other materials to investigate, such as the long-term effect of radiation on concrete, which is poorly understood, DOE's Szilard said.

"We'll be looking for mechanisms that have perhaps not manifested itself up to now," he said.

The industry has become skilled at replacing even large, expensive components like reactor heads and steam turbines -- in overhauls that can cost several hundred million dollars. Once flaws are identified, nearly any structural problem can be solved.

"Today, virtually every component in a reactor plant has been replaced at one point," said Tiffany Edwards, a DOE spokeswoman. "The exceptions are the reactor pressure vessel and the concrete [containment] structures. However, even those could be considered."

The pressure vessel remains the largest challenge for scientists as they try to determine which types of vessel might not make it to 80 years. If underlying flaws are found, there is the possibility that a metallurgical technique called annealing, employed in the past on nuclear reactors in Russia but never in the United States, could be used.

Put simply, annealing requires heating the metal of the pressure vessel to remove the damage created by radiation, possibly restoring it close to its original condition. "It's a very big job and very challenging," Was said.

"There are questions of how quick the damage will come back," he said. "Not all of these questions have been answered, but there's encouragement that even the pressure vessel could be replaced."

The United States has been well served by the caution of engineers who built the country's first generations of nuclear power plants, as seen in the ability of its plants to seamlessly cross the 40-year mark, MAI's Lee said.

"We know that with the margins that have been taken into account, they were so much over-designed that it is no problem to go beyond 40 years," Lee said.

While ultimately the decision of when to extend the lives of nuclear power plants will come down to utilities, Gaertner expects that those decisions will not be held back by structural problems.

"We feel pretty confident that there are technical solutions to all the issues," he said, "and that the cost will probably be worth it."

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